In a conventional electrophotographic printer, as shown in the external view of FIG. 10, a print image is obtained by converting an electrical signal into a light signal, using the light signal to form an electrostatic latent image on a photoconductive member which has previously been charged by a corona discharge, developing this image, and transferring it to an output medium. For example, there are electrophotographic printers having an electroluminescent imaging head (electroluminescent printer head) 52 and a photosensitive drum 51.
The exposure system of such a conventional electrophotographic printer uses for example a thin-film electroluminescent array which is small, has stable luminance and is capable of quick response.
FIG. 11 shows a transverse cross-section of an electroluminescent imaging head, comprising an array of electroluminescent elements 71 and a rod lens array 72. The array of electroluminescent elements 71 comprises the following electrodes and layers formed on a substrate 81, in this order: a metal electrode 82, an insulating layer 83, a light emitting layer 84, an insulating layer 85 and a transparent electrode 86.
The rod lens array 72 is positioned so that the whole light emitting portion is covered. A photoreceptor 90 and the array of electroluminescent elements 71 are positioned on opposite sides of rod lens array 72 (For example, the above described characteristics are described in Japanese unexamined patent application number Sho 62-92868.) The light emitting portion of each electroluminescent elements, as shown in the plan view of FIG. 12, is separately formed with a dimension L in the slow scan direction and a spacing P in the fast scan direction.
It has been proposed, as the exposure system for an electrophotographic printer, as shown in the external view of FIG. 13 to use an electroluminescent imaging head consisting of an electroluminescent device which uses edge emission of electroluminescent elements 92 formed on a substrate 91. (See "The Construction and Characterization of A 400-dpi Thin-film Electroluminescence Edge Emitter" D. Leksell, et al, Proceedings of the SID, Vol. 29/2, 1988, pp. 147-150)
Next, a conventional method of driving an electroluminescent imaging head is described, referring to FIG. 14 and FIG. 15. FIG. 14 is a circuit block diagram of the conventional electroluminescent imaging head and FIG. 15 is a timing chart of the drive operation.
In the circuit of the electroluminescent imaging head, as shown in FIG. 14, each of the drivers IC1 to ICm is connected to electroluminescent elements CEL, and the other ends of the electroluminescent elements CEL are connected to a common drive signal COM. Each driver IC consists of a shift register 1 which reads a data signal together with a clock signal, latch circuit 2 which latches the data signal under the control of a latch signal and output circuit 3 which controls output under the control of an output enable signal OE by changing the level of the latched data signal.
As illustrated in FIG. 15, the data signal is output under the control of the output enable signal OE from output circuit 3, to form an output signal On, which has different waveforms in the light emitting and non-light emitting states. The common drive signal COM has waveforms synchronized with the light emitting and non-light emitting waveforms of the output signal On. Light is emitted when and only when the potential difference of the output signal On and the common drive signal COM is sufficiently high.
In more detail, the data signal undergoes level conversion in output circuit 3 and the output signal On has a peak value V.sub.MOD. The common drive signal COM has a waveform with a positive peak value V.sub.POS which is equal to the sum of V.sub.TEL, the light-emitting threshold voltage of the electroluminescent elements, and the voltage V.sub.MOD ; the waveform has a negative peak value V.sub.NEG, which is equal to -V.sub.TEL. The potential difference between the output signal On and the common drive signal COM is applied to the electroluminescent elements CEL. Light is emitted when and only when the potential difference between the output signal On and the common drive signal COM is over the threshold. Thus, electroluminescent elements in the electroluminescent imaging head are controlled to emit light or not, as required.
The distribution characteristics of the amount of light applied to the photosensitive drum, which rotates at a constant rate, when writing a single pixel using the electroluminescent imaging head, are, as shown in FIG. 16, trapezoidal, and almost rectangular in the fast scan direction, they are, as shown in FIG. 17, asymmetrical in the slow scan direction, rising to a peak, and then falling more gradually. This asymmetry produces distortion in the formation of pixels, and means that high quality printing in an electrophotographic printer is not possible.
The light distribution in the slow scan direction as shown in FIG. 17 results from the relative movement of the electroluminescent imaging head and the photosensitive drum and the light emission response characteristics of the electroluminescent elements, which have a short rise time but a decay time as long as 1 ms.